Conventionally, for dental diagnoses, X-ray imaging apparatuses, intraoral cameras, dental cameras, X-ray CT, MRI, etc. have been used in order to image the stomatognathic region.
Images obtained with X-ray imaging apparatuses ultimately are transmitted images, and the information of the subject along the traveling direction of X rays is detected in an overlapped manner. Therefore, the internal structure of the subject cannot be known in a three-dimensional way. In addition, since X rays are harmful to the human body, the annual exposure dose is limited, and they can be handled only by qualified experts. Moreover, they can be used only in chambers surrounded by shielding members such as lead and lead glass.
Intraoral cameras image only the surface of intraoral tissues, and the internal information such as the information about a tooth therefore cannot be obtained. Like X-ray imaging apparatuses, X-ray CT is harmful to the human body. Moreover, it has low resolution, and involves large and expensive apparatuses. MRI has low resolution, and involves large and expensive apparatuses. Moreover, it cannot image the internal structure of a tooth containing no moisture.
Incidentally, an optical coherence tomography apparatus (hereinafter, referred to as “OCT apparatus”) is harmless to the human body, and can obtain the three-dimensional information of subjects with high resolution. Therefore, it is applied in the field of ophthalmology, such as for tomographic measurements of a cornea or a retina, (for example, see JP2003-329577A, JP2002-310897A, JP11-325849A, and JP2001-059714A). It should be noted that OCT is the abbreviation for optical coherence tomography. The optical coherence tomography apparatus also may be called an optical interference tomography apparatus.
Here, a conventional OCT apparatus will be described. FIG. 30 is a diagram showing the configuration of a conventional OCT apparatus. In an OCT unit 1 constituting the OCT apparatus shown in FIG. 30, the light emitted from a light source 2 is collimated by a lens 3, and then split into reference light 6 and measuring light 5 by a beam splitter 4. The measuring light 5 passes through a galvano mirror 8, and is focused by a lens 9 on a measured object 10, where the measuring light 5 is scattered and reflected. Thereafter, the measuring light 5 again passes through the lens 9, the galvano mirror 8 and the beam splitter 4, and is focused on a photodetector 14 by a lens 7. Meanwhile, the reference light 6 passes through a lens 12, is reflected at a reference mirror 13, and passes through the lens 12 and the beam splitter 4 again. Thereafter, the reference light 6 overlaps the measuring light 5, enters the lens 7, and is focused on the photodetector 14.
The light source 2 is a low-temporal-coherence light source. Those components of light emitted from a low-temporal-coherence light source at different time points tend not to interfere with one another. Therefore, an interference signal will appear only when the distance of the optical path through which the measuring light 5 passes is substantially equal to the distance of the optical path through which the reference light 6 passes. Accordingly, the reflectance distribution of the measured object 10 in the depth direction (the z axis direction) can be obtained by measuring the intensity of the interference signal with the photodetector 14, while moving the reference mirror 13 in the direction of the optical axis of the reference light 6 and thus changing the difference in the optical path length between the measuring light 5 and the reference light 6. That is, the configuration of the measured object 10 in the depth direction can be determined by sweeping the optical path length difference.
The measuring light 5 reflected in the z axis direction at the measured object 10 carries the object information of the measured object 10 in the waveform of its electromagnetic wave. However, there is no photodetector capable of directly measuring the waveform on the temporal axis, because the optical waveform of the measuring light 5 is a phenomenon that is very rapid. Therefore, the OCT apparatus causes the measuring light 5 reflected at the measured object 10 and the reference light 6 to interfere with each other, thereby converting the reflection property information of each area of the measured object 10 into a change in the intensity of the interference light. As a result, the photodetector 14 can perform the detection on the temporal axis.
A two-dimensional cross-sectional image of the measured object 10 can be obtained by performing scanning in the transverse direction (the x-axis direction) with the galvano mirror 8, in addition to the scanning in the depth direction (the z-axis direction) of the measured object with the reference mirror 13. With such an OCT apparatus, measurement can be performed with high resolution in the order of several micrometers. Accordingly, with the OCT apparatus, a high resolution image of the interior of a living body can be obtained in a nondestructive and contactless manner.
With regard to the application of the OCT apparatus to the field of the dentistry, examples are disclosed in which tomographic images of teeth are taken using OCT apparatuses (for example, see Documents 1 to 5 below).    Document 1: LASER KENKYU, October 2003: Technical development of the optical coherence tomography centering on medical science    Document 2: Journal of Biomedical Optics, October 2002, Vol. 7 No. 4: Imaging caries lesions and lesion progression with polarization sensitive optical coherence tomography    Document 3: APPLIED OPTICS, Vol. 37, No. 16, and 1 Jun. 1998: Imaging of hard-and soft-tissue structure In the oral cavity by optical coherence tomography    Document 4: OPTICS EXPRESS, Vol. 3, No. 6, and 14 Sep. 1998: Dental OCT    Document 5: OPTICS EXPRESS, Vol. 3, No. 6, and 14 Sep. 1998: In vivo OCT Imaging of hard and soft tissue of the oral cavity